In order to maintain a sterile environment, C-arms and/or their tracking attachments are conventionally covered with tubular or pouch-shaped films. There are a number of drapes for C-arms that are used to sterilely cover various parts of the C-arm apparatus, including, for example, the image intensifier. These drapes generally comprise an inner side that faces the part to be covered (e.g., the C-arm or its parts, such as a cylindrical tracking attachment with reflector marker attachments) and an outer, sterile side that faces the sterile environment of the operating theater.
Conventional C-arm drapes are known, for example, from U.S. Pat. No. 6,697,664 and from U.S. Pat. No. 5,802,719. In U.S. Pat. No. 5,802,719, the drape, which may cover an inner part of the C-arm, is flexible and may be drawn into shape by a number of clips that can be attached to the frame of the C-arm. A drape for shielding x-rays from being scattered by the operating table is known from U.S. Pat. No. 6,481,888.
The known covering films may be manufactured from a relatively thin material. This may be done deliberately so as to enable the drapes to remain flexible and to be easily draped over objects. However, conventional drapes are so flexible that they often form creases and plaits (also referred to as pleats) when they are drawn over an object. Such creases and plaits can become a problem when the object to be covered is to be detected by a tracking system.
Optical tracking systems typically receive either actively emitted light (e.g., light flashes), which in most cases are in the infrared spectrum, or light passively reflected off of specialized reflector markers. Tracking attachments for the image intensifiers of C-arms, for example, may have a number of such reflector markers on their outer surface (the surface may have a cylindrical periphery). If these markers are then covered with a film that has creases and plaits and, for example, if a crease or plait is situated over a reflection marker, its reflection image can be distorted (scattered). This can result in a camera tracking system incorrectly detecting the position of the marker in three-dimensional space, which in turn can distort registration of the x-ray recordings produced using the C-arm.